Although a mobile radio communication device (such as a mobile phone), which is a mobile radio station, communicates with a station via radio waves when a call is placed with the mobile radio communication device, the radio waves emitted from the sending antenna of the station reach the receiving antenna of the mobile radio communication device after being blocked, diffracted, and reflected by various surrounding buildings and terrain roughness. That is to say, a so-called multi-path propagation path is constructed between the mobile radio communication device and the station. In such a mobile radio communication environment, when a call is placed with a mobile radio communication device while traveling on board of an automobile or the like, the phase and amplitude of the radio wave vary with the travel of the mobile radio communication device, causing phasing.
Usually, since a modulation scheme for adjusting the phase and amplitude of a carrier wave is used for a radio wave signal used for communication with the mobile radio communication device serving as a mobile radio station, variation in the phase and amplitude due to this phasing may cause the mobile radio communication device to fail to receive data, or to receive wrong data, resulting in a malfunction. The station may encounter similar problems.
As a solution of this phasing, there is a method of inserting a pilot symbol whose phase and amplitude are known, into a data symbol, and sending the symbol. This method is a method whereby the known pilot symbol is sent from a sending side to a receiving side, the phasing variation received on a radio propagation path is estimated (channel estimation) on the receiving side, and based on the estimated channel estimate, the phase and amplitude of the data symbol are reversed by the amount of variation received on the radio propagation path to eliminate the effect of the phasing.
Conventionally, as a method of inserting a pilot symbol into a data symbol, as disclosed in Japanese Patent Laid-Open No. 2004-07793 (Document 1), and Japanese Patent Application No. 2004-015819 (Document 2), a time multiplexing scheme whereby a pilot symbol is inserted between data symbols, and a parallel scheme whereby a pilot symbol is inserted in parallel with a data symbol are known.
FIG. 10 shows a symbol sequence in the time multiplexing scheme, where pilot symbol blocks . . . P1, P2, . . . are placed in the data symbol blocks . . . D1, D2, . . . in a time-shared manner, and transmitted. FIG. 11, which shows a symbol sequence in the parallel scheme, shows a pilot symbol sequence P transmitted in parallel with the data symbol sequence D.
In FIG. 10, although each of the pilot symbol blocks indicated by P1 and P2 constitutes a block containing a plurality of pilot symbols displaying data 1 and 0, the number of pilot symbols within one block is not limited to two or more, and may be one pilot symbol. Each of the data symbol blocks . . . D1, D2 . . . also constitutes a block containing a plurality of data symbols displaying data 1 and 0. One slot is constituted by the pilot symbol block P1 and the data symbol block D1.
In FIG. 11, each of the pilot symbol P and the data symbol D constitutes a plurality of pilot symbol lines (pilot symbol sequence) and a plurality of data symbol lines (data symbol sequence).
As a parallel scheme, there are many related arts whereby a weight is assigned to a pilot symbol in response to phasing variation, and averaging is performed. For example, those related arts have been disclosed in Japanese Patent Laid-Open No. 2004-007793 (Document 1), and Japanese Patent Application No. 2004-015819 (Document 2).
In addition, related art whereby the number of pilot symbols is controlled in response to movement velocity of a mobile phone has been disclosed in Japanese Patent Laid-Open No. 2001-127692.
However, in all of the above related arts, the number of pilot symbols for calculating a channel estimate has a maximum value with a fixed upper limit, and averaging is performed based on whether or not to assign a weight to the fixed number of pilot symbols; therefore, channel estimate that takes into account the fixed number or more pilot symbols could not be determined.
Usually, for slow phasing, when a lot of pilot symbols are averaged, the accuracy of channel estimation is improved. Meanwhile, for rapid phasing, if averaging is performed beyond the phasing variation period, the rapid phasing cannot be reflected correctly; on the contrary, the accuracy of channel estimation drops. Accordingly, it is known that, for rapid phasing, a small number of pilot symbols are averaged to improve the accuracy of channel estimation.
With a method using weighting of the above related art, rapid phasing can be handled, but the fixed number or more pilot symbols cannot be used for slow phasing because the number of pilot symbols has a maximum value with a fixed upper limit. Particularly, phasing variation little occurs when a mobile radio communication device does not travel, and remains in the same place; there is a request to further improve the accuracy of the channel estimate in such a state.
Thus, for example, a large upper limit can be considered in advance for the number of pilot symbols used for channel estimation; however, simply increasing the upper limit of the number of pilot symbols increases the amount of calculation in cases where weighting is performed, while in cases where weighting is not performed, the accuracy is degraded in rapid phasing environment due to the number of pilot symbols used for channel estimation being fixed at the maximum value, which are problems.